1. Field of the Invention
The Present Invention is related to a torque-limiting slip mechanism for an underbody spare tire carrier.
2. Description of the Related Art
Spare tire carriers for use with the underbody of vehicles are well known. These types of carriers have been developed to include a winch-based mechanism that is used to raise and lower a tire received in the underbody carrier. The winch includes a first end that includes a tire holder and a second end that engages a lift drum. The wire is passed through a central opening of the tire and the holder engages the tire underside. Thus, as an individual, using an appropriate tool that engages the lift drum, rotates the lift drum about a fixed axis, the wire is wound or unwound about the lift drum, thereby lifting or lowering the tire. When the tire is properly lifted to its stored orientation within the carrier, it is sandwiched between the vehicle underside and the tire holder and prevented from movement even during substantial vehicle vibration.
In practice, prior art units have been subject to undesirable failure. What happens is that the winch mechanism is used to lift a tire from the ground to its stored orientation within the carrier. However, an individual using the winch is unable to determine when tire is properly stored. Thus, in accordance with the appropriate desire to make sure that the tire holder properly traps the tire within the carrier, there is a tendency to continue to exert torque upon the lift drum in an attempt to further rotate the wire about the lift drum, placing ever-increasing tension on the wire after the tire is totally stored. While some tire compression is appropriate, placing undue tension on the wire may cause it to snap, resulting in the tire dropping to the ground and carrier failure.
Complicated overrunning clutches have been devised to prevent additional lift drum rotation once a predetermined maximum tensile stress has been placed on the wire. In practice, after a maximum acceptable tension has been placed on the wire, additional torque application will result in the clutch overrunning to prevent additional tension from being placed on the wire. However, such clutches are complex, expensive to manufacture, heavy, and take up extremely valuable vehicle packaging space. The clutch must be substantially redesigned depending on critical tire characteristics (e.g., weight, diameter, and tire thickness). Even when properly designed, prior art clutches have a tendency to adversely react to long-term detrimental environmental conditions including rusting of iron-based components or increased brittleness of plastic based components that may freeze up or lock the carrier in its locked orientation. Carrier failure will not be discovered until a flat tire requires the use of carrier. Under such circumstances, a high stress situation is significantly heightened.
In an attempt to simplify the use of complicated overrunning clutches, an attempt has been made to develop a slip mechanism. In one known slip mechanism a clutch plate includes a pair of diametrically opposed ears that project axially into curved portions of a pair of arcuate leaf-type metal springs having opposing hooked shape end portions. The springs are carried by a clutch drive plate having a pair of generally T-shaped cavities for receiving and retaining the end portions of the springs. The clutch drive plate has a center hole that receives a mating hub portion of a drive shaft so that the clutch plate is positively driven by the drive shaft. When excessive torque is applied to the drive shaft, the ears ideally cam the springs inwardly. In practice, however, the ears have an undesirable tendency to either bend outwardly in response to the biasing force of the springs or even break off, resulting in slip mechanism failure. Even when operational, a jarring force intensified response is received by carrier operator as the slip mechanism is activated by the curved portions of the springs abruptly engaging the ears, suggesting carrier failure even as the slip mechanism is being operated.
In an alternative approach, the ears are replaced by openings in the clutch plate. The openings facilitate the entry of contaminants into the slip mechanism while not eliminating the jarring force intensified response.
One further alternative approach has a plurality springs rigidly connected at opposing ends to fixed receiving pockets of the clutch plate. A central portion of each spring selectively engages a cam lobe connected to the drive shaft. When slip is required, the wire springs deform outwardly around the cain lobes and permit the clutch plate to rotate relative to the clutch cam. The jarring force intensified force still results. Moreover, as the cam lobes selectively and abruptly engage the wire springs, spring fatigue or pocket failure may prevent proper slip mechanism operation and carrier failure.
Thus, for the types of known slip mechanisms that have been developed as an alternative to overrunning clutches, the springs are rigidly secured at opposing ends, and forced to selectively and abruptly engage an opposing structure, resulting in either abrupt spring contraction or failure of the opposing structure. When the spring abruptly contracts, the carrier operator experiences a jarring force that is disconcerting to the carrier operator, and may result in an a feeling that the carrier has somehow failed rather than a realization that the override slip mechanism has been used to minimize such failure. Several of these types of mechanisms are also prone to inappropriate environmental contamination.
The present invention is directed to a simplified slip mechanism for use with an underbody tire carrier, which comprises a camshaft, a tire carrier drive and a plurality of free-floating non-rigidly secured spring elements disposed between the camshaft and the tire carrier drive. An individual applies torque through the camshaft. The spring elements prevent relative rotation between the camshaft and the drive such that the drive and the spring elements rotate through the same angular extent as the camshaft when a torque is applied to the camshaft unless a pre-determined torque is exceeded. Thus, a tire disposed between a tire holder hanging from a wire and the underbody of a vehicle may be raised or lowered as the wire is wound or unwound through the rotation of the carrier drive. However, once the tire is appropriately stored, the application of any additional torque by the camshaft upon the carrier drive is prevented by the elastic deformation of the spring elements to permit relative rotation between the camshaft and carrier drive.
The camshaft includes an outer section, a central section, and an inner section, the central section including a plurality of mating surfaces and intermediate segments. The carrier drive includes a pocket adapted to receive the central section of the camshaft, the pocket having an interior wall. The spring elements are disposed between the central section and the interior wall. The elements are preferably arcuate with an interior concave surface of a central body engaging a mating surface and free ends engaging the interior wall. When there is no torque applied, the spring elements are automatically centered. However, when a torque is applied that is greater than a pre-determined torque, the spring elements elastically deform with respect to the difference in surface dimension and characteristics between the mating surfaces and intermediate segments as the camshaft continues to rotate.
The slip mechanism of the present invention is deceptively simple. It takes key advantage of having free-floating spring elements to promote both normal drive and slip as varying torque conditions require. It is also extremely compact, taking up little if any additional room within the carrier. When an overriding torque is applied, the mechanism activates smoothly while still providing an appropriate resistance that indicates that input torque is no longer required to complete tire storage within the carrier. It is also very easy to adjust the inventive mechanism for different tire types through the use of different spring elements or the interaction between the camshaft, the spring elements, and the interior wall.